Electrolytic process of stripping metallic coatings from a ferrous metal base



Patented Jan. 8, 1952 ELECTROLYTIC PRObESS OF STRIPPING METALLIC COATINGS FROM A FERROUS METAL BASE Grayland T. Larsen, Wyandotte, Mich., assignor to General Motors Corporation, Detroit, Mich, a corporation of Delaware No Drawing. Application March 11, 1947, Serial No. 733,978

7 Claims.

This invention relates to an electrolytic process of stripping a metallic coating such as copper, nickel or chromium (or combinations of these metals) from a ferrous metal surface.

There are numerous instances in which it is necessary or desirable to strip a metallic coating such as copper, nickel, or chromium (or cn1binations of these metals) from a ferrous metal surface such as steel. Various methods have been proposed heretofore for stripping a metal coating of this sort. In some instances mechanical polishing has been employed. In other cases chemical or electrochemical stripping has been proposed. The prior methods of which I am aware are subject to one or more disadvantages. The primary object of the present process is a new and improved electrolytic process of stripping such coating from a ferrous metal surface.

One electrolytic stripping treatment proposed heretofore is to employ an alkaline sodium ni trate stripping bath. In the past the operation of this type bath has been complicated by etching of the base metal (steel or other ferrous metal). This trouble has been particularly objectionable in the case of freshly prepared baths. I have found that the etching tendency of the sodium nitrate type stripping bath is attributable to sodium chloride present as a normal impurity in the sodium nitrate employed in making up the bath. For example, in a stripping bath employing four pounds of commercial sodium nitrate per gallon of solution there will be present as a normal impurity about one-fourth of an ounce of sodium chloride. I have discovered that the etching tendency of this type of electrolytic stripping bath can be eliminated by presence in the bath of the nitrites of any one of sodium, potassium or ammonium.

A typical electrolytic stripping bath in accord anoe with the invention consists of four pounds of sodium nitrate (containing one-fourth ounce of sodium chloride as normal impurity), oneeighth ounce of sodium hydroxide and threefourths ounce of sodium nitrite per gallon of water. In operation this bath is maintained at a temperature of 160 to 170 F. for best results. However, satisfactory stripping can be obtained over the temperature range of 100 to 220 F.

The metal to be stripped is made the anode the bath. Current densities within the range of 50 to 150 amperes per square foot and higher have been found suitable. The higher current .denstties are preferred. The passage of current through the bath is continued until all of metal coating is stripped from the ferrous metal base. The normal stripping time at a current density of about 100 amperes per square foot runs about thirty minutes with work having the following average plate. thickness chromiu 0.00001"; copper 0.0008"; and nickel 0.0007" with a total plate thickness of 0.00151".

Considerable variation is permissible in the composition of the electrolytic stripping bath. Baths have been used successfully in which the commercial sodium nitrate varied from two pounds per gallon of water up. to saturation conditions. Satisfactory performance has been obtained with a variation in the sodium nitrite concentration from about one-half to about four ounces per gallon. The sodium hydroxide may range from about one twentieth to about one-half ounce per gallon; however, the higher amounts are unnecessary. After the bath is made up it is not necessary to make further additions of sodium hydroxide because caustic is formed within the bath by cathode reactions. After extended operation of the bath it is necessary to neutralize the excess alkalinity so developed by the addition of nitric acid in order to maintain optimum stripping performance. Observation. of the stripped base metal will indicate when such addition should be made. When a normal brown stain, which results after stripping, is not removed by a dip in 50% muriatic acid at room temperature, the solution pH is too high and nitric acid should be added. Normal addition to an 800 gallon tank is two gallons of nitric acid. Should etching begin to-appear on the base metal a deficiency of sodium nitrite is indicated and should be corrected by addition of more of the sodium nitrite.

In place of sodium nitrite in the above path, equivalent amounts of the nitrites of potassium or ammonium may be used. Potassium or ammonium hydroxide may be employed in place of sodium hydroxide in making up the stripping bath. The nitrates of potassium or ammonium also may be used in place of sodium nitrate. The nitrates of potassium and ammonium also contain chloride as an impurity and the addition of alkali nitrites to electrolytic stripping baths containing these nitrates eliminates etching of the base metal caused by the chloride impurity. From the standpoint of cost and commercial availability, the sodium salts or compounds arepreferred. Wherever the term alkali metal is used it is to be understood that the ammonium radical is the functional equivalent and is to be included.

After stripping, the parts are rinsed in cold running water to remove scum and then dipped in 50% muriatic acid at room temperature until a brown stain remaining from the stripping treatment is completely removed. The parts are then thoroughly rinsed in cold running water.

Various changes and modifications in the embodiments of the invention described herein may the class consisting of copper, nickel and chromium from a ferrous metal base without etching the base metal which comprises, passing electric current from said coated base as anode through a stripping solution consisting essentially of about at least two pounds of a commercial alkali metal nitrate containing an alkali metal chloride as a normal impurity, about one-eighth ounce of alkali chloride being present in each two pounds of said commercial alkali nitrate, about one-half ounce to about four ounces of an alkali metal nitrite and about one-twentieth to one-half ounce of an alkali metal hydroxide per gallon of water until said coating is stripped from the ferrous metal base, said solution temperature being within the range of about 100-220 F. during said stripping operation.

2. A process of electrolytically stripping a metal coating consisting of at least one metal of the class consisting of copper, nickel and chromium from a steel base without etching the basic metal which comprises passing electric current at a current density of about 100 amperes per square foot from the coated base as anode through a solution consisting essentially of about four pounds of commercial sodium nitrate containing as a normal impurity about one-fourth ounce of sodium chloride, one-half to about four ounces of sodium nitrite, and about one-eighth ounce sodium hydroxide per gallon of water until said metal coating is stripped from the steel base, said solution being within the temperature range of about 160-170 F. during said stripping treatment.

3. In a process of electrolytically stripping a metal coating consisting of at least one metal of the class consisting of copper, nickel and chromium from a ferrous metal base by passing an electric current from said coated ferrous metal base as anode at a current density of at least 50 amperes per square foot through an aqueous solution consisting essentially of at least about two pounds per gallon of a commercial alkali metal nitrate containing an alkali metal chloride as a normal impurity in the approximate proportion of one-eighth ounce alkali metal chloride in each two pounds of said alkali metal nitrate and about one-twentieth to one-half ounce of an alkali metal hydroxide maintained at a temperature within the range of about 100 to 220 F., the improvement which consists in adding to said solution about one-half to about four ounces of an alkali metal nitrite per gallon of solution.

4. In a process of electrolytically stripping a metal coating consisting of at least one metal of the class consisting of copper, nickel and chromium from a ferrous metal base by passing electric current from said coated ferrous metal base as anode at a current density within the range of 100 to 150 amperes per square foot through an aqueous solution consisting essentially of about four pounds of commercial sodium nitrate having about one-fourth ounce of sodium chloride as a normal impurity and about one-twentieth to one-half ounce of an alkali hydroxide and maintained during operation at a temperature of 160 to 170 F., the improvement which consists in adding to said solution sodium nitrite within the approximate limits of one-half to four ounces per gallon.

5. A process of electrolytically stripping a metal coating consisting of at least one metal of the class consisting of copper, nickel and chromium from a steel base without etching the steel base which comprises passing electric current from said coated steel base as anode at 2. current density within the approximate range of to amperes per square foot through an electrolyte consisting essentially of an aqueous solution of the following ingredients in approximately the proportions specified per gallon of water:

63.75 ounces NaNOs 0.25 ounce NaCl 0.5 to 4.0 ounces NaNOz 0.05 to 0.5 ounce NaOH peres per square foot through an electrolyte consisting essentially of an aqueous solution of the following ingredients in approximately the proportions specified per gallon of water:

31.875 to 63.75 ounces NaNOa NaCl in the proportion of 0.125 ounce for each 31.875 ounces NaNOa 0.5 to 4.0 ounces NaNOz 0.05 to 0.5 ounce NaOH said solution being maintained during operation at a temperature of about to F.

7. A process of electrolytically stripping a metal coating consisting of at least one metal of the class consisting of copper, nickel and chromium from a steel base without etching the steel base which comprises passing electric current from said coated steel base as anode at a current density within the approximate range of 100 to 150 amperes per square foot through an electrolyte consisting essentially of an aqueous solution of the following ingredients in approximately the proportions specified per gallon of water:

63.75 ounces NaNOs 0.25 ounce NaCl 0.75 ounce NaNOz 0.125 ounce NaOH said solution being maintained during operation at a temperature of about 160 to 170 F.

GRAYLAND T. LARSEN.

REFERENCES CITED The following references are of record in the file of this patent:

FOREIGN PATENTS Country Date Germany Aug. 18, 1939 OTHER REFERENCES The Monthly Review of The American Electroplaters Society, July 1945, pp. 672, 673.

Number Metal Cleaning and Finishing, November 1933, pp. 465. 466. V

Comptes Rendus (Doklady) de lacademie des Sciences de lU. R. S. 8., vol. 14, No.5, (1937), pp. 295 thru 298.

The Manufacture of Nitric Acid and Nitrates, by A. Cottrell (1923), vol. 6, p. 32. 

1. A PROCESS OF ELECTROLYTICALLY STRIPPING A METAL COATING CONSISTING OF AT LEAST ONE METAL OF THE CLASS CONSISTING OF COPPER, NICKLE AND CHROMIUM FROM A FERROUS METAL BASE WITHOUT ETCHING THE BASE METAL WHICH COMPRISES, PASSING ELECTRIC CURRENT FROM SAID COATED BASE AS ANODE THROUGH A STRIPPING SOLUTION CONSISTING ESSENTIALLY OF ABOUT AT LEAST TWO POUNDS OF A COMMERCIAL ALKALI METAL NITRATE CONTAINING AN ALKALI METAL CHLORIDE AS A NORMAL IMPURITY, ABOUT ONE-EIGHTH OUNCE OF ALKALI CHLORIDE BEING PRESENT IN EACH TWO POUNDS OF SAID COMMERCIAL ALKALI NITRATE, ABOUT ONE-HALF OUNCE TO ABOUT FOUR OUNCES OF AN ALKALI METAL NITRITE AND ABOUT ONE-TWENTIETH TO ONE-HALF OUNCES OF AN ALKALI METAL HYDROXIDE PER GALLON OF WATER UNTIL SAID COATING IS STRIPPED FROM THE FERROUS METAL BASE, SAID SOLUTION TEMPERATURE BEING WITHIN THE RANGE OF ABOUT 100*-220* F. DURING SAID STRIPPING OPERATION. 